The investigator's laboratory has discovered an appalling prevalence of malsynchronization of circadian rhythms among volunteers averaging 70 years of age. Aging people may be abnormally resistant to the circadian synchronizing effects of bright light. If so, we may need to develop alternative methods by which older Americans can synchronize their circadian rhythms to the environment. Recent studies indicate that exercise may shift circadian rhythms in young adults, but nothing is known about the value of exercise for regulating circadian rhythms in the aging population. It seems crucial to extend our understanding of exercise effects on the circadian system and, specifically, to compare the potential values of exercise and bright light for correcting the circadian malsynchronization of older Americans. The project will establish circadian phase response curves both for exercise and for bright light in 96 volunteers, ages 18-30 plus 128 older volunteers ages 60-75 years. Volunteers will be recorded for 4.8 to 4.6 days while following an ultra-short sleep-wake cycle, consisting of 30 minutes for sleeping, followed by 60 minutes for waking. Baseline circadian phases of urinary 6-sulphatoxymelatonin, urinary free cortisol, temperature and sleep propensity will be assessed every 90 min. Oral temperature will be sampled every 30 min. Volunteers will be given experimental phase-shifting treatments (exercise or bright light) for 3 days. Resultant circadian phases will then be determined to compute the phase response curves, in order to examine interactions of stimulus (exercise vs. light), age, and gender on circadian responsiveness.